x86/mm: Fix pti_clone_pgtable() alignment assumption

Guenter reported dodgy crashes on an i386-nosmp build using GCC-11
that had the form of endless traps until entry stack exhaust and then
#DF from the stack guard.

It turned out that pti_clone_pgtable() had alignment assumptions on
the start address, notably it hard assumes start is PMD aligned. This
is true on x86_64, but very much not true on i386.

These assumptions can cause the end condition to malfunction, leading
to a 'short' clone. Guess what happens when the user mapping has a
short copy of the entry text?

Use the correct increment form for addr to avoid alignment
assumptions.

Fixes: 16a3fe634f6a ("x86/mm/pti: Clone kernel-image on PTE level for 32 bit")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20240731163105.GG33588@noisy.programming.kicks-ass.net

diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index 2e69abf4f852abb7d8e29c7673dc82f685cf7b89..48c503208c794d0ba25d52036f9a9cfe42201d01 100644 (file)
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -374,14 +374,14 @@ pti_clone_pgtable(unsigned long start, unsigned long end,
                         */
                        *target_pmd = *pmd;
 
-                       addr += PMD_SIZE;
+                       addr = round_up(addr + 1, PMD_SIZE);
 
                } else if (level == PTI_CLONE_PTE) {
 
                        /* Walk the page-table down to the pte level */
                        pte = pte_offset_kernel(pmd, addr);
                        if (pte_none(*pte)) {
-                               addr += PAGE_SIZE;
+                               addr = round_up(addr + 1, PAGE_SIZE);
                                continue;
                        }
 
@@ -401,7 +401,7 @@ pti_clone_pgtable(unsigned long start, unsigned long end,
                        /* Clone the PTE */
                        *target_pte = *pte;
 
-                       addr += PAGE_SIZE;
+                       addr = round_up(addr + 1, PAGE_SIZE);
 
                } else {
                        BUG();